The invention relates to processing of radio frequency signals, particularly to the balancing of signals.
Transformers are usually used for the balancing and phasing of radio frequency signals. Transformers wound on a ferrite core perform well at low frequencies. When the frequency increases the characteristics of the ferrite core deteriorate, so that ceramic bodies are typically used as transformer cores in the 900 MHz frequency range, for instance.
Wound transformers are expensive, however, and therefore also strip line transformers according to FIG. 1a are used at high frequencies. Regarding the balancing characteristics at high frequencies a strip line transformer performs at least as well as wound transformers, and a strip line transformer is also very cheap to manufacture. The strip line transformer according to FIG. 1a is described in more detail in the Finnish patent publication No. 91930, or in the corresponding British patent application publication GB-9203902.3.
FIGS. 1b and 1c show examples of some other prior art structures realised with strip lines. The structures of FIG. 1b and 1c are band-pass filters. It is typical for these structures that the length of the strips is determined according to the operating frequency of the structure, whereby the length of the structure can not be freely selected. Other structures realised with strip lines, such as filters and directional couplers of other types, are described in the books Leo Young, "Microwave Filters Using Parallel Coupled Lines", Artech House, Dedham, Mass. 1972, and Matthaei, Young, Jones, "Microwave Filters, Impedance-Matching Networks and Coupling Structures", Artech House, Dedham, Mass. 1980.
However, there are certain problems with the strip line transformers. The relatively large area on a printed circuit board required by a strip line transformer has many inconvenient effects. The signal strength radiated by a member formed by strip lines on a printed circuit board is directly proportional to the member's size, so the signals appearing in a strip line transformer are easily coupled by radiation to other stages of the device, and correspondingly, signals from other stages are easily coupled to the strip line transformer. Mounting errors in other parts of the devices will also easily influence the characteristics of a strip line transformer due to the large area of a strip line transformer. A typical mounting error causing problems in connection with strip line transformers is RF shielding cover askew: because the area of the strip line transformer is rather large the distance to the skew RF shielding cover will vary, when measured at different positions of the strip line transformer. This has an influence i.a. on the balance characteristics of the strip line transformer.
An object of the invention is to realise a phasing and balancing member, which has a smaller size than prior art solutions. An object of the invention is also to realise a phasing and balancing member, which causes less high frequency radiation than a strip line transformer. A further object of the invention is to realise a phasing and balancing member, which is less sensitive to external interference signals than a strip line transformer.